Lubricating oils used in internal combustion engines such as automobile engines require heat and oxidation stability that allows them to withstand harsh conditions for prolonged periods. Base oils with high viscosity indexes have been desired in recent years from the standpoint of achieving fuel savings, and various additives and base oils have been investigated. For example, it is common to include, as additives in base oils, peroxide-decomposable sulfur-containing compounds such as zinc dithiophosphate or molybdenum dithiocarbaminate, or ash-free antioxidants such as phenol-based or amine-based antioxidants (for example, see Patent documents 1-4).
Known processes for improving the viscosity-temperature characteristic/low-temperature viscosity characteristic and thermal oxidation stability include processes in which feedstock oils containing natural or synthetic normal paraffins are subjected to hydrocracking/hydroisomerization to produce high-viscosity-index base oils (see Patent documents 5-6, for example). Methods for improving the low-temperature viscosity characteristics of lubricating oils also exist, wherein additives such as pour point depressants are added to highly refined mineral oil-based base oils.    [Patent document 1] Japanese Unexamined Patent Application Publication HEI No. 4-36391    [Patent document 2] Japanese Unexamined Patent Application Publication SHO No. 63-223094    [Patent document 3] Japanese Unexamined Patent Application Publication HEI No. 8-302378    [Patent document 4] Japanese Unexamined Patent Application Publication HEI No. 9-003463    [Patent document 5] Japanese Patent Public Inspection No. 2006-502298    [Patent document 6] Japanese Patent Public Inspection No. 2002-503754